Clear air turbulence significantly affects the comfort of passengers on commercial aircraft and has even caused some would be passengers to forego flying due to their fears associated with the turbulence. Because clear air turbulence can occur with little or no warning, the passengers tend to stay in their seats with their seat belts fastened. At times, though, every passenger must get up for comfort and physiological reasons. Therefore, if the aircraft must proceed through the turbulence, it would be useful if the aircrew could alert the passengers to the disturbance before the aircraft encounters it.
Preferably, the aircraft would avoid the turbulence altogether but even that preventative measure requires that the turbulence be detected or predicted before it occurs. While much turbulence (e.g. the turbulence associated with thunderstorms) can be predicted or detected, clear air turbulence can not be detected or predicted by currently available technology. The reason that clear air turbulence cannot be detected is that it consists of masses of air having slightly different temperatures, pressures, and densities moving at various speeds and directions in the atmosphere. The minute differences in these air masses do not reflect radar differently enough to make the radar return from one mass of air distinguishable from the radar return from another mass of air.
While meteorological maps provide flight crews some indication of where turbulence might be expected, these maps are not perfect. First, they tend to become stale within hours and are based on underlying meteorological models that are far from perfect also. Additionally, turbulence occurs across a wide variety of geometric scales. Some turbulent areas can extend for many kilometers, or even hundreds of kilometers (e.g. the turbulent region surrounding the jet stream). Other areas of turbulence occur on the scale of kilometers or fractions of kilometers such as the turbulence associated with the downstream side of a mountain that is subjected to brief wind gusts of significant velocity. Due to their scale, these smaller volumes of turbulence will not appear on the meteorological maps.
In the absence of any better approach, the aviation industry has created a system in which the pilots of each aircraft radio in reports of the clear air turbulence that they encounter on their routes, or “airways.” Subsequent aircraft flying the same airway can maneuver in response to these reports but risk encountering turbulence along their detour. Obviously, the first aircrew to fly along a given airway after the airway has been vacant for some time will have no reports on which to base evasive action. Likewise, those aircraft on unplanned detours such as when an airport is too busy to accept arrivals, or is otherwise shut down (by for example severe weather), will have no way to foresee the turbulence along the route.